SSBU!
So, apparently, it is possible to have a 0-second (almost), 1-player match in SSBU.
I was playing on For Glory 1 vs 1. I was on the pre-battle "training stage". The other player selected their character and their status became OK. Then, after the confirmation sound and while the screen was going black, the other player disconnected (I saw their box shifting away).
I then entered a 1-player battle, which immediately ended with a victory.
I was able to save and view the replay without any issue. I even uploaded it for reference: look for Italy / For Glory 1 vs 1 / Yoshi's Island Omega / Ryu in the replay finder.
Would it be worth noting? I give anyone full permission to upload the video on Youtube or the like / reuse any part of it, if deemed relevant.